1. Field of the Invention
The present invention is directed to an apparatus and method for transmitting messages between a plurality of subscriber stations. In particular, the present invention is directed to transmitting information between subscriber stations by means of modulated carrier signals. A subscriber station having a message transfer installation that is operated in accordance with this invention comprises a transmitter that is connected to a voice current circuit, the output of which is connected to a transmission path, and a receiver that is connected to an audio current circuit, the input of which is connected to the transmission path.
2. Discussion of Background and Other Information
A message transmission installation is disclosed in European Patent B1-0 078 714, in which subscriber stations communicate with each other by means of a two-wire line. The subscriber stations are electrically connected to the two-wire line so that information can be transferred over the two-wire line using a frequency-modulated carrier, a pair of predetermined frequencies being used for each transmission direction. In addition to the establishment of connections between two subscriber stations, this installation permits the establishment of conference call connections, in which two or more subscriber stations are simultaneously connected to each other.
In such connections, message transmissions on the two-wire line are not made using a carrier frequency band signal, but rather by the use of an unmodulated baseband signal. Such an arrangement has several disadvantages. For instance, such a system permits unauthorized persons to listen in on a conference call, simply by connecting their own subscriber stations to the line.
A second disadvantage of such a system is that it permits only one conference call at any one time.
Another disadvantage of such a system is that distant conference call participants sound weaker than those participants that are less distant.